Much stronger measures are needed to halt the growing obesity epidemic, according to public health academics.

They say the Coalition is failing to act because it fears being seen to impose "nanny state" policies.

In 2008 some 26 per cent of British men and women were obese, but that could rise to 46 per cent by 2030 if historical trends continue, according to work published in The Lancet today (FRI). That would mean an additional 11 million obese people in less than 20 years.

Academics say this is worrying because obese people tend to suffer much more from diabetes, heart disease and cancer.

Besides the rising human cost, that would also put additional strain on the NHS at a time when the population is ageing fast as well.

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By 2050 obesity could cost the state £32 billion a year - almost a third of today's NHS budget.

At the moment there is no VAT on most foodstuffs. Any move to change that would be politically fraught, particularly as it would hit poorer homes hardest.

But Prof Steven Gortmaker, from the Harvard School of Public Health, said taxing unhealthy food and drink would save governments billions by reducing obesity-related illness, as well as bringing in revenue.

He calculated that an unhealthy food tax would save California, with a population of 37 million, $1.5 billion (about £1 million) a year.

His analysis showed a 'fat tax' was the single most effective measure, in terms of lives saved, followed by traffic light food labelling, and tighter control of junk food advertising to children.

Such moves were "effective and cost-effective to society", he said.

Be he lamented: "So far, governments haven't shown any leadership whatsoever. We have let the market do its work and it's worked well to produce obesity."

Politicians preferred to use less controversial but less effective options, he said, like paying for obesity surgery and healthy eating promotions.

Prof Klim McPherson from Oxford University, who worked on the obesity forecasts, criticised Coalition ministers for believing they could solve the problem without drastic action.

He believed Conservative ministers did understand the scale of the issue, but argued they shied away from taking the necessary action because they did not want to be seen acting like a nanny state.

"They feel that they don't want to be labelled with that particular insult," he said. "They feel enfeebled by their ideology".

He continued: "I think they do 'get it', but they don't know what to do about it and that's what needs to change."

He argued politicians need not fear instigating unhealthy food taxes.

"These taxes have the effect of bringing in income, but also saving a lot of money for electors," he said.

"I don't think people will throw politicians out for bringing in these taxes, as long as they understand what the issues are," he said.

The two were speaking ahead of publication of a series of articles in The Lancet today (FRI), detailing the scale of the global obesity problem and asking how it should be tackled.

The prospect of Britain taxing unhealthy foods, as Denmark has done, remains unlikely.

David Cameron firmly believes people can be "nudged" to better health by creating incentives to persuade them to make better choices.

He also favours working with businesses rather than imposing legislation or raising taxes.

Last autumn Andrew Lansley, the Health Secretary, said that "rather than nannying people we will nudge them".

He added: "Nudges are very important. Tax is not a nudge, tax is a shove."

Many would feel a food tax - even on unhealthy products - to be a tax on living. Deciding what or how to tax would also be a minefield.

The food industry is against the idea.

Andrew Opie, food director at the British Retail Consortium, said: "There 's no such thing as an unhealthy food just an unhealthy diet. Demonising particular foods is not the answer. The tax regime already discriminates against many processed foods by subjecting them to VAT.

"Retailers are actively improving diets through education and providing healthier eating choices because that's what will make a difference.

"New taxes would just push up prices for hard - pressed households."

Anne Milton, the Health Minister, said: "We have no current plans to impose a 'fat tax', but we are working with food companies to reduce fat, sugar and salt and ensure healthier options are available."

She added: "We will be saying more about our plans to tackle obesity in a new document later this year."